Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2017-1130 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 12 December 2017 c Author(s) 2017. CC BY 4.0 License.
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Large contribution of fossil-fuel derived secondary organic
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carbon to water-soluble organic aerosols in winter haze of China
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Yan-Lin Zhang1,2,3*, Imad El-Haddad3, Ru-Jin Huang3,4*, Kin-Fai Ho4,5, Jun-Ji Cao4*,
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Yongming Han4, Peter Zotter3, #, Carlo Bozzetti3, Kaspar R. Daellenbach3, Jay G. Slowik3, Gary
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Salazar2, AndréS.H. Prévôt3*, Sönke Szidat2*
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and Technology, 210044 Nanjing, China
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2
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University of Bern, 3012 Bern, Switzerland
Yale-NUIST Center on Atmospheric Environment, Nanjing University of Information Science Department of Chemistry and Biochemistry & Oeschger Centre for Climate Change Research,
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Academy of Sciences, 710061 Xi’an, China
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China
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*
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[email protected] (Y.-L.Z.);
[email protected] (A. Prévôt);
[email protected]
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(R.-J.H.);
[email protected] (J.J.C.);
[email protected] (S.S.).
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Phone: +86 25 5873 1022; fax: +86 25 5873 1193
Paul Scherrer Institute (PSI), 5232 Villigen, Switzerland Key Laboratory of Aerosol Chemistry and Physics, Institute of Earth Environment, Chinese
School of Public Health and Primary Care, The Chinese University of Hong Kong, Hong Kong,
To whom correspondence should be addressed. E-mail:
[email protected] or
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Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2017-1130 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 12 December 2017 c Author(s) 2017. CC BY 4.0 License.
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Abstract
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Water-soluble organic carbon (WSOC) is a large fraction of organic aerosols (OA) globally and
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has significant impacts on climate and human health. The sources of WSOC remain very
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uncertain in polluted regions. Here we present a quantitative source apportionment of WSOC
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isolated from aerosols in China using radiocarbon (14C) and offline high-resolution time-of-
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flight aerosol mass spectrometer measurements. Fossil emissions on average accounted for 32-
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47% of WSOC. Secondary organic carbon (SOC) dominated both the non-fossil and fossil
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derived WSOC, highlighting the importance of secondary formation to WSOC in severe winter
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haze episodes. Contributions from fossil emissions to SOC were 61±4% and 50±9% in
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Shanghai and Beijing, respectively, significantly larger than those in Guangzhou (36±9%) and
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Xi’an (26±9%). The most important primary sources were biomass burning emissions,
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contributing 17-26% of WSOC. The remaining primary sources such as coal combustion,
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cooking and traffic were generally very small but not negligible contributors, as coal
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combustion contribution could exceed 10%. Taken together with earlier
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apportionment studies in urban, rural, semi-urban, and background regions in Asia, Europe and
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USA, we demonstrated a dominant contribution of non-fossil emissions (i.e., 75±11%) to
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WSOC aerosols in the North Hemisphere; however, the fossil fraction is substantially larger in
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aerosols from East Asia and the East Asian pollution outflow especially during winter due to
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increasing coal combustion. Inclusion of our findings can improve a modelling of effects of
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WSOC aerosols on climate, atmospheric chemistry and public health.
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C source
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Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2017-1130 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 12 December 2017 c Author(s) 2017. CC BY 4.0 License.
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1 INTRODUCTION Water-soluble organic carbon (WSOC) is a large fraction of atmospheric organic
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aerosols (OA), which contributes approximately 10% to 80% of the total mass of organic carbon
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(OC) in aerosols from urban, rural and remote sites (Zappoli et al., 1999;Weber et al.,
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2007;Ruellan and Cachier, 2001;Wozniak et al., 2012;Mayol-Bracero et al., 2002). Only 10 to
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20% of total mass of WSOC has been resolved at a molecular level, and it consists of a large
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variety of chemical species such as mono- and di-carboxylic acids, carbohydrate derivatives,
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alcohols, aliphatic and aromatic acids and amino acids (Fu et al., 2015;Noziere et al., 2015).
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Recent studies suggest that the water-soluble fraction of HUmic LIke Substances (HULIS) is
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a major component of WSOC, which exhibits light-absorbing properties (Limbeck et al.,
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2005;Andreae and Gelencser, 2006;Laskin et al., 2015). Therefore, WSOC has significant
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influences on the Earth’s climate either directly by scattering and absorbing radiation or
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indirectly by altering the hygroscopic properties of aerosols and increasing cloud condensation
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nuclei (CCN) activity (Asa-Awuku et al., 2011;Cheng et al., 2011;Hecobian et al., 2010).
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WSOC can be directly emitted as primary particles mainly from biomass burning
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emissions or produced from secondary organic aerosol (SOA) formation (Sannigrahi et al.,
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2006;Kondo et al., 2007;Weber et al., 2007;Bozzetti et al., 2017b;Bozzetti et al., 2017a).
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Ambient studies provide evidence that SOA formation through the oxidation of volatile organic
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compounds (VOCs) and gas-to-particle conversion processes may be a prevalent source of
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WSOC (Kondo et al., 2007;Weber et al., 2007;Miyazaki et al., 2006;Hecobian et al., 2010).
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WSOC is therefore thought to be a good proxy of secondary organic carbon (SOC) in the
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absence of biomass burning (Weber et al., 2007). By contrast, water-insoluble OC (WIOC) is
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thought to be mainly from primary origins with a substantial contribution from fossil fuel
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emissions (Miyazaki et al., 2006;Zhang et al., 2014b).
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Due to a large variety of sources and unresolved formation processes of WSOC, their
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relative fossil and non-fossil contributions are still poorly constrained. Radiocarbon (14C)
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analysis of sub-fractions of organic aerosols such as OC, WIOC and WSOC enable an
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Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2017-1130 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 12 December 2017 c Author(s) 2017. CC BY 4.0 License.
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unambiguous, precise and quantitative determination of their fossil and non-fossil sources
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(Zhang et al., 2012;Zhang et al., 2014b;Zhang et al., 2014c). Meanwhile, the application of
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aerosol mass spectrometer measurement and positive matrix factorization and multi-linear
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engine 2 (ME-2) can quantitatively classify organic aerosols into two major types such as
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hydrocarbon-like OA (HOA) from primary fossil-fuel combustion and oxygenated organic
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aerosol (OOA) from secondary origin (Zhang et al., 2007;Jimenez et al., 2009). Field
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campaigns with the aerosol mass spectrometer (AMS) have revealed a predominance of OOA
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in various atmospheric environments, although their sources remain poorly characterized
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(Zhang et al., 2007;Jimenez et al., 2009). Previous studies found OOA is strongly correlated
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with WSOC from urban aerosols in Tokyo, Japan, the Pearl River Delta (PRD) in South China
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and Helsinki, Finland, indicating similar chemical characteristics, sources and formation
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processes of OOA and WSOC (Kondo et al., 2007;Xiao et al., 2011;Timonen et al., 2013).
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Similarly, HOA is mostly water insoluble and the major portion of water insoluble OC (WIOC)
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can be assigned as HOA (Kondo et al., 2007;Daellenbach et al., 2016). Therefore,
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measurement of WIOC and WSOC aerosols may provide new insights into sources and
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formation processes of primary and secondary OA, respectively, which also will elucidate the
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origin of HOA and OOA as measured by AMS (Zotter et al., 2014b).
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C
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In this paper we apply a newly developed method to measure 14C in WSOC of PM2.5
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(particulate matter with an aerodynamic diameter of small than 2.5 μm) samples collected at
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four Chinese megacities during an extremely severe haze episode during winter 2013 (Zhang
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et al., 2015;Huang et al., 2014). In conjunction with our previous dataset from the same
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campaign, we quantify fossil and non-fossil emissions from primary and secondary sources of
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WSOC and WIOC. The dataset is also complemented by previous
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apportionment studies conducted in urban, rural and remote regions in the North Hemisphere
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to gain an overall picture of the sources of WSOC aerosols.
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2 MATERIALS AND METHODS
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C-based source
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Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2017-1130 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 12 December 2017 c Author(s) 2017. CC BY 4.0 License.
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2.1 Sampling
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During January 2013 extremely high concentrations of 24-h PM2.5 (i.e. often >100
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µg/m3) were identified in several large cities in East China (Huang et al., 2014;Zhang et al.,
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2015). To investigate sources and formation mechanisms of the haze particles, an intensive
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field campaign was carried out in four large cities, Beijing, Xi’an, Shanghai and Guangzhou,
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which are representative cities of the Beijing-Tianjin-Hebei region, central-northwest region,
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Yangtze Delta Region, and Pearl River Delta Region, respectively . The sampling procedures
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have been previously described in detail elsewhere (Zhang et al., 2015). Briefly, PM2.5 samples
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were collected on pre-baked (450 °C for 6 hours) quartz filters using high-volume samplers for
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24 h at a flow rate of ~1.05 m3/min from 5 to 25 January 2013. The sampling sites in each city
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were located at campuses of universities or at research institutes, at least 100 m away from
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major emission sources (e.g., roadways, industry and domestic sources). One field blank sample
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for each site was collected and analyzed. The results reported here were corrected for these field
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blanks (Zotter et al., 2014a;Cao et al., 2013). All samples were stored at -20 °C before analysis.
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The PM2.5 mass was gravimetrically measured with an analytical microbalance before and after
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sampling with the same conditions (~12 hour)
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2.2 OC and EC mass determinations
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A 1.0 cm2 filter punches were used for OC and EC mass determination with a OC/EC
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analyzer (Model4L) using the EUSAAR_2 protocol (Cavalli et al., 2010). The replicate analysis
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(n=6) showed an analytical precision with relative standard deviations smaller than 5%, 10%,
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and 5% for OC, EC and TC, respectively. The field blank of OC was on average 2.0 ± 1.0
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µg/cm2 (equivalent to ~0.5 μg/m3), which was used for blank correction for OC. EC data was
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not corrected for field blank, because such a blank was not detectable.
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2.3 Offline-AMS measurement and PMF source apportionment
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The water-soluble extracts from the same samples were analyzed by a high resolution time of
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flight aerosol mass spectrometer (HR-ToF-AMS) and the resulting mass spectra were used as
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Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2017-1130 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 12 December 2017 c Author(s) 2017. CC BY 4.0 License.
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an inputs for positive matrix factorization (PMF) for the source apportionment of the WSOC,
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OC and PM2.5. The methodology applied, and the AMS-PMF results obtained are detailed in
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Huang et al. (2014) and will only be briefly described in the following.
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Filter punches (the equivalent of ~4 cm2) were sonicated in 10 mL ultrapure water (18.2 MΩ
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cm at 25 °C, TOC
